VOP_ADVISE(9)

💖 VOP_ADVISE 💖

okay bestie, so this one lives in section 9 of the Kernel Developer's Manual — it's VOP_ADVISE, and it's the vnode operation that gives the kernel a little heads up about how a file's data is gonna get used. it's giving "hey, just so you know" energy. 🦋

✨ NAME ✨

VOP_ADVISE — apply advice about use of file data 💅

✨ SYNOPSIS ✨

#include <sys/param.h>
#include <sys/vnode.h>

int
VOP_ADVISE(struct vnode *vp, off_t start, off_t end, int advice);

✨ DESCRIPTION ✨

so this call applies advice for a range of a file's data. it's the behind-the-scenes queen that implements the posix_fadvise system call. 🎀

and here's the cast of arguments, bestie:

• vp — the vnode of the file. the main character, literally.
• start — the start of the range of file data.
• end — the end of the range of file data. a value of OFF_MAX means "apply the advice all the way to the end of the file," periodt.
• advice — the type of operation to apply to the file data. possible values are:
  • POSIX_FADV_WILLNEED — initiate an asynchronous read of the file data if it's not already resident. like pre-gaming the read, iykyk. 🌸
  • POSIX_FADV_DONTNEED — decrease the in-memory priority of clean file data, or discard clean file data. it's giving "we're done here, no cap." 💀

if the start and end offsets are both zero, then the operation should be applied to the entire file. 🌈

okay and this is the important part, don't skip it: this call is advisory only. it may perform the requested operation on a subset of the requested range — including not performing it at all — and still return success. so don't come for it when it doesn't do the thing. it literally warned you. 🫶

✨ LOCKS ✨

the file should be unlocked on entry. yes bestie, unlocked. don't be holding anything when you walk in. 🔓

✨ RETURN VALUES ✨

zero is returned if the call is successful. otherwise, an appropriate error code is returned. that's it, that's the vibe check. ⭐

✨ ERRORS ✨

• [EINVAL] — an invalid value was given for advice. 🙈

✨ SEE ALSO ✨

• vnode(9) 💜

and that's the whole page, babe. this one came out of the Debian edition of the manual, dated September 26, 2015 — so it's a lil vintage, but she still slays. 🎀✨ bye bestie, go advise some files responsibly! 👑